To prevent a sheet from crumpling by providing a head pressing mechanism for setting a nip width at the central part of a thermal head equal to or wider than that at the opposite ends thereof.

A single leaf spring 5 is pressed against the central part of a thermal head 1 thus pressing the thermal head 1 against a platen 2. The leaf spring 5 is brought into direct contact with an aluminum heat plate 1b. The leaf spring 5 touches the aluminum heat plate 1b at a position 5a on the extension of a line connecting between the center of the platen 2 and a nip part 12. Since the thermal head 1 is deformed while copying the platen 2, the nip width at the central part of the platen 2 is set equal to or wider than that at the opposite ends thereof. Consequently, the speed of the sheet at the central part can be set equal to or higher than that at the opposite ends thereof. Since a speed difference appears on a thermal sheet 7 from the central part toward the outside thereof, a force for spreading the thermal sheet 7 is generated and the sheet 7 is prevented from crumpling.